I have rad that the maximum number of slots in PC wermacth is 25, is this the same in more complex campagins/scenarios like Berlin or Barbarrossa? Can you handle more units there? Furthermore, what is the scale of the units? REgiments/divisions? depends on the scenario?

I recommend you go to the editor and check the scenario of interest. It will tell you the number of units allowed and show you their locations. you can even change the number, locations and type units. Also. I have found that from the ranges of the artillery, the most appropriate scale for (starting) 10 strength units are regiments.